The invention relates to a hob cutter with a coating and a method for coating a hob cutter. Milling cutters, for example hob cutter tools, are subjected in practice to high stresses. As a result, there is comparatively high wear. Hob cutter tools, in particular, are subject to strong strike or impact stresses during operation.
It is known to provide such tools with a coating that is adapted to their application conditions, for example a hard coating. For this, physical vapor phase deposition (PVD) is used. For hobbing and for other cutting work, thus far, mainly single layer coatings, e.g., TiN, TiAlN or AlCrN coatings have been used. With such coatings, there is a constant chemical composition across the thickness. There is a conflict of goals in that on the one hand the hardest possible coatings are desired for the wear resistance, however, such hard layers can absorb mechanical stress peaks only to a small degree. Such peak stresses occur, for example, during process steps with discontinuous cuts. Through this, cracks can propagate through the tool coating, and lead to tool failure. From WO 2004/059030 A2, the entire contents of which is incorporated herein by reference, a workpiece with a hard coating containing AlCr, and a method for its production are known. There, so-called gradient layers can be provided with increasing Al content towards the surface. The layer composition can change continuously or step-wise over the layer thickness.
The known coatings have in common that they do not always possess optimal properties with regard to their wear resistance on the one hand, and their sensitivity to strike or impact stress on the other. The removal of swarf from a workpiece being processed represents another problem. For optimizing the removal of swarf, it is known, for example for arc PVD coatings to smooth the surface of the coating through finishing treatment. However, this represents an additional work step and gives rise to corresponding costs.
For the production of tool coatings, it is known from WO 2004/059030 A2, the entire contents of which is incorporated herein by reference, to vaporize the metallic portion of the hard coating from alloyed sources (so-called targets) with respectively constant compositions (e.g., Al:Cr compositions). However, due to the constant composition of the target, the flexibility with regard to the possible composition of layers that can be deposited is limited. With highly stressed tools, such as hob cutters, the coating loses its function due to wear after a certain service life. In order to avoid having to replace the entire tool in such a case, it is desired to renew just the coating, thus, to carry out a recoating. For this, at least parts of the tool surface that have worn out coating must be removed, for example, abraded. Subsequently, the tool can be recoated. However, the surface areas not removed in the process, have sometimes been subjected to very high temperatures in the prior use of the tool. Due to the change effected thereby of these areas, this leads to an at least partially insufficient adhesion of the new coating. This in turn, leads to the fact that the coating can fail during operation, for example, detach. Due to this problem also, the number of possible recoatings is limited.
Starting from the described state of the art, the objective of the invention is to provide a hob cutter tool and method of the initially named type, wherein the wear resistance on one hand, and the sensitivity against strike and impact stress on the other hand are improved in a manner that is simple to produce, and the ability to be recoated is improved.